Generally, a direct bonding method for wafers is known as a high integration method for a semiconductor device. In the direct bonding method, wafers are directly bonded to each other by using organ/inorganic adhesives or without using the adhesives in a method of manufacturing a SOI (Silicon On Insulator) or forming P-N junction by combining a P type wafer with a N type wafer.
In the method using organic/inorganic adhesives, the wafers can be bonded at a low temperature, but the method is not widely used because of interfacial stress and thermally or chemically unstable adhesive layer.
In addition, in the wafer bonding method, cleaning and surface treatment are performed to reduce bonding defects before bonding the wafer. However, because the cleaning and the surface treatment are separately performed, there is a problem that the process yield is reduced.In the U.S. Pat. No. 4,939,101, there is disclosed a method that wafers are contacted with each other and then directly bonded to each other by heating them at a temperature higher than 1,100° C. under a pressure of about 1,000 to 15,000 psi. However, there are problems that the temperature is too high and thus, the method can be used when the bonded material cannot endure the temperature, and thermal mismatching is caused when the bonded materials are different from each other.
On the other hand, in the wafer bonding method, generally, planarization process of wafer surface is performed for reducing bonding defects before bonding the wafers. A chemical mechanical polishing (CMP) is usually used as the planarization method. For performing the CMP process, polishing is performed in a separate CMP process device and then the wafer bonding process is performed and thus, the processes should be separately performed. Accordingly, the process efficiency is reduced and a pollution problem is caused by slurry used in the CMP process. In addition, surface steps remain even after completion of polishing, thereby causing bonding defects in bonding of the final wafer.